The inkjet printing industry uses various types of inks, such as oil-based inks, solvent-based (non-aqueous) inks, water-based inks, and solid inks (which are melted in preparation for dispensing). Solvent-based inks are fast drying, and as a result, are widely used for industrial printing. When solvent-based inks containing binders and other ingredients are jetted onto a substrate, the solvent(s) partially or fully evaporate from the ink, leaving the binder and other ingredients such as pigment particles on the printed substrate in the form of a dry film. During the drying process, the solvents, which are often volatile organic compounds (VOC), emit vapors, and therefore can pollute the environment. The amount of pollution produced can increase greatly with higher printing speeds or for wide format images, where large amounts of ink are deposited onto a substrate. As a result of this and other concerns, efforts related to preparing inks that are environmentally friendly have moved some research in the direction of water-based inks. However, radiation-curable (or photon-curable) water-based ink compositions are noticeably limited among available options due to their specific formulation properties. Accordingly, the development of radiation radiation-curable water-based ink compositions that exhibit, when printed, specific desirable printing properties such as, for example, jetting properties as well as improved adhesion would be an advancement in the field of inkjet technology.
Ultraviolet (UV) curable ink can be cured after printing by application of UV radiation or light. Typically, UV curable inks include monomers that form polymers by free radical polymerization. The growing end of each polymer chain is a radical that reacts with additional monomers, transferring the radical to the end of the chain as each monomer is added. A photo initiator is used to form the first radicals to begin the polymerization process. The photo initiator is capable of absorbing UV light to generate radicals to react with the monomers.
Two types of photo initiators can be used in UV curable compositions. Type I photo initiators are unimolecular photo initiators that undergo a hemolytic bond cleavage upon absorption of UV light, forming radicals. Type II photo initiators are bimolecular photo initiators. These are used as a system of a photo initiator with a synergist, which can together form radicals upon exposure to UV light. Some type II photo initiators react by hydrogen abstraction from the synergist to the photo initiator.